The tag lives in the <head>: <meta name="description" content="…"/>. Google truncates it at roughly 155–160 characters on desktop and 120 on mobile, so the first sentence has to earn the click.
A good meta description restates the page's value proposition, includes the primary keyword naturally, and ends with a light call to action ('Compare 12 tools', 'Free and no signup').
Search engines rewrite meta descriptions about 60 % of the time when the query doesn't match. That's fine — writing a strong description still wins the other 40 % of impressions.